1. Field of the Invention
The present invention relates to an apparatus and a method for processing signals, a recording medium, and a program and, in particular, to an apparatus and a method for processing signals, a recording medium, and a program capable of outputting a natural sounding voice even when a packet to be received is lost.
2. Description of the Related Art
Recently, IP (Internet protocol) telephones have attracted attention. IP telephones employ VoIP (voice over Internet protocol) technology. In this technology, an IP network, such as the Internet, is employed as part of or the entirety of a telephone network. Voice data is compressed using a variety of encoding methods and is converted into data packets. The data packets are transmitted over the IP network in real time.
In general, there are two types of voice data encoding methods: parametric encoding and waveform encoding. In parametric encoding, a frequency characteristic and a pitch period (i.e., a basic cycle) are retrieved from original voice data as parameters. Even when some data is destroyed or lost in the transmission path, a decoder can easily reduce the affect caused by the loss of the data by using the previous parameters directly or after some process is performed on the previous parameters. Accordingly, parametric encoding has been widely used. However, although parametric encoding provides a high compression ratio, parametric encoding disadvantageously exhibits poor reproducibility of the waveform in processed sound.
In contrast, in waveform encoding, voice data is basically encoded on the basis of the image of a waveform. Although the compression ratio is not so high, waveform encoding can provide high-fidelity processed sound. In addition, in recent years, some waveform encoding methods have provided a relatively high compression ratio. Furthermore, high-speed communication networks have been widely used. Therefore, the use of waveform encoding has already been started in the field of communications.
Even in waveform encoding, a technique performed on the reception side has been proposed that reduces the affect caused by the loss of data if the data is destroyed or lost in a transmission path (refer to, for example, Japanese Unexamined Patent Application Publication No. 2003-218932).